qemu-char: socket backend: disconnect on write error

Socket backend read handler should normally perform a disconnect, however
the read handler may not get a chance to run if the frontend is not ready
(qemu_chr_be_can_write() == 0).

This means that in virtio-serial frontend case if
 - the host has disconnected (giving EPIPE on socket write)
 - and the guest has disconnected (-> frontend not ready -> backend
   will not read)
 - and there is still data (frontend->backend) to flush (has to be a really
   tricky timing but nevertheless, we have observed the case in production)

This results in virtio-serial trying to flush this data continiously forming
a busy loop.

Solution: react on write error in the socket write handler.
errno is not reliable after qio_channel_writev_full(), so we may not get
the exact EPIPE, so disconnect on any error but QIO_CHANNEL_ERR_BLOCK which
io_channel_send_full() converts to errno EAGAIN.
We must not disconnect right away though, there still may be data to read
(see 4bf1cb0).

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Daniel P. Berrange <berrange@redhat.com>
CC: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1486045589-8074-1-git-send-email-den@openvz.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index 4068dc5..865c527 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -97,6 +97,9 @@ static gboolean tcp_chr_accept(QIOChannel *chan,
                                GIOCondition cond,
                                void *opaque);
 
+static int tcp_chr_read_poll(void *opaque);
+static void tcp_chr_disconnect(Chardev *chr);
+
 /* Called with chr_write_lock held.  */
 static int tcp_chr_write(Chardev *chr, const uint8_t *buf, int len)
 {
@@ -114,6 +117,13 @@ static int tcp_chr_write(Chardev *chr, const uint8_t *buf, int len)
             s->write_msgfds_num = 0;
         }
 
+        if (ret < 0 && errno != EAGAIN) {
+            if (tcp_chr_read_poll(chr) <= 0) {
+                tcp_chr_disconnect(chr);
+                return len;
+            } /* else let the read handler finish it properly */
+        }
+
         return ret;
     } else {
         /* XXX: indicate an error ? */